Avalanche Advisory - Tue, Feb. 07, 2017

THIS ADVISORY EXPIRED ON February 9, 2017 @ 6:55 amAvalanche Advisory published on February 7, 2017 @ 6:55 am

Issued by
Doug Lewis - Eastern Sierra Avalanche Center

bottom line:

Primary concern for the next 48 hours:

Wind Slab – sensitive Wind Slabs will likely be encountered on NW-N-NE-E-SE aspects in the mid to upper elevations below ridgelines and in and around terrain features that promote drifting and crossloading. The strong winds may form Wind Slabs extending downslope more than usual and in unusual areas (especially in the low and mid elevations). Anticipate fragile cornices along ridgelines. Natural avalanches likely; human- triggered avalanches very likely. Travel in avalanche terrain is not recommended. Numerous large avalanches possible. Avoid wind loaded terrain and runout zones below wind-loaded terrain.

Wind Slab avalanches release naturally during wind events and can be triggered for up to a week after a wind event. They form in lee and cross-loaded terrain features. Avoid them by sticking to wind sheltered or wind scoured areas.

With the additional snow and strong Southwesterly winds forecasted to impact the area from today (Tuesday) into Wednesday, sensitive Wind Slabs will form on NW-N-NE-E-SE aspects in the mid to upper elevations below ridgelines and in and around terrain features that promote drifting and crossloading. The strong winds may form Wind Slabs extending downslope more than usual and in unusual areas (especially in the low and mid elevations). Anticipate fragile cornices along ridgelines. Avoid slopes steeper than 30 degrees, leeward convexities, slopes below ridgelines, and the side-walls of gullies Additional concern is for potential whiteout conditions with limited visibility Tuesday.

Storm Slab avalanches release naturally during snow storms and can be triggered for a few days after a storm. They often release at or below the trigger point. They exist throughout the terrain. Avoid them by waiting for the storm snow to stabilize.

The weekend’s lower density snowfall has been buried by wet dense snow over the past 24 hours as temperatures have climbed forming an upside down snowpack structure with the potential for Storm Slabs on all aspects in protected areas in the low to mid elevations as snowfall continues to accumulate. Snow surface cracking are possible clues for Storm Slabs. Travel in or around avalanche terrain is not recommended.

Avalanche Character 3: Loose Wet

Loose Wet avalanches occur when water is running through the snowpack, and release at or below the trigger point. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.

Another moderate atmospheric river (AR) cycle began to impact the region Thursday thru Friday with 6” to 10” of relatively low-density new snow reported throughout the forecast region. This was accompanied by moderate to strong localized Southwesterly winds in the mid to upper elevations forming Wind Slabs on exposed NW-N-NE-E-SE aspects. The snow bonded relatively well to the old snow interface and provided some excellent ski conditions. However, a second warmer wave moved through the region Monday with dense snow and strong Southwesterly winds depositing dense snow layer over lower density snow (upside-down snowpack). Temperatures continued to climb throughout Monday and are forecasted to rise further into Tuesday night with additional dense snow being deposited above ~8500’. This combination is conducive for Storm Slab instability for all aspects and elevations, especially above 8500’. This second round of precipitation will be accompanied by strong SW winds, which will form new sensitive Wind Slabs on NW-N-NE-E-SE aspects in the mid to upper elevations below ridgelines and in and around terrain features that promote drifting and crossloading. Wind Slabs may extend downslope more than usual and in unusual areas (especially in the low and mid elevations). The latest AR event is accompanied by strong warm air advection aloft raising snow levels to near 8000-9000 feet Tuesday. Temperatures will continue to rise through Wednesday with rain possible below~8500’. As the lower elevation snowpack becomes saturated by rain and melt water, Loose Wet avalanches are possible.

Caution – Snow creep cracks have been observed throughout the forecast area with some large enough to swallow a person. They have been found at all elevations on steep slopes, base of cliffs, couloirs, and below buried rock bands. This latest round of wet snow/rain may cause these to become more pronounced.

Lakes Basin, Mammoth Lakes (2/6/17) - Toured from end of Old Mammoth Rd, across Lake Mary, and up Lake Mary Ridge. Lots n lots o' wind in all but the most sheltered spots. Mostly from SW, but at times swirling. See attached photo of wind ridges on "groomed" cross country track ... maybe not the best skate ski day;-). Cross-country track on north side of Lake Mary had 5' tall wind drifts. Snowpit dug 3/4way up Lake Mary Ridge, 9,200' in one of the more sheltered form the wind locations we could find. See attached profile. ECTN results. (tests not propagating). 2pm = -1deg C. Handpits, CT tests, ECT tests done on leeward side of Lake Mary ridgetop all failed either upon isolation or with easy force, and failed cleanly. See attached video. Lots of subtle wind slab layers in upper 40cm. Shooting cracks on this same aspect just on leeward side of ridge went about 3.5m (11.5’) in front of skis, and 5m (16’) behind skis with jump on slope. Slope probably would have avalanched had it been steeper for a greater distance before mellowing out below. Dense layer of snow on top makes skiing a bit funky even in sheltered areas.

Yost Lake, June Mtn. (2/5/17) - Clouds, gusty winds and a moderate wave of precipitation ahead of approaching storm system shut down J-7 mid-day. Toured out from the ski area to assess how much the previous day snow stability. Moderate to strong Southwesterly winds at ridgetop, observed snow banners on many peaks with significant transport above treeline. The soft slab noted the day previously had settled (previously Fist minus, now fist) and the shear note 5 to 10" from surface had strengthened from easy shears to moderate. Ski cuts on convex rolls produced 1 very small pocket ~6'x6' that dribbled to a stop and limited sluffing where disturbed. Snow depth below 8000' ~ 3- 4'. No recent avalanches observed but visibility limited due to clouds and snow. Snow below ~8000 noticeably heavier than previous day.

White Wing, June Mtn. (2/5/17) - Wind loading occurring on N and NE slopes especially filling in gullies, some cracking occurred NNW and NW slopes creating very small isolated pockets of hard slabs (about 6inch crowns) at about 9400 feet. No other stability concerns were seen. Skiing was very good on NE slopes and poor/variable on W and NW slopes. Hard blue ice present in pockets on the ridge of white wing above the tree line.

Fern Lake, June Mountain (2/5/17) - Toured from Double Eagle up Four Seasons to above Fern Lake, 7200' to 10200'. Original objective was North side of Carson but deferred to plan B due to increasing wind transport on ridges and decreasing visibility. Surface snow notably stiffer than yesterday's on the Four Seasons slope but still held soft snow on protected north aspects. Skied 2 NW rock-lined chutes that still held good wintery snow, though hand pits found a stiff, 1F+ wind slab that ranged from 10 to 15cm in thickness, up to 30cm in isolated areas Hand shears easy. Noted 2 naturals that slid recently originating in rock bands just above Devil's Slide, D1, 1.5' crowns. Snow much wetter and heavier below 8000'.

weather

… Wind Advisories and the High Wind Warnings for the region through 10 PM PST Tuesday evening…

Tuesday thru Wednesday –The Sierra Crest has received around 1-2 inches of liquid precipitation, with localized areas up to 3 inches. The forecast for the next 48 hours is for the Sierra Crest to receive an additional 4-6 inches of precipitation, with a major portion of that coming in the next 12-18 hours. The extreme depth of the moisture coupled with strong 70+ knot ridgetop winds are expected to push significant precipitation into the Sierra foothills. The spillover precipitation is expected to intensify through midday before stronger shadowing again takes over this afternoon. Winds are forecast to become much stronger today (Tuesday) reaching 70+ kts this afternoon with gusts of upwards of 150 mph.

Thursday thru Friday - storm is on track with high winds possible Thursday morning through evening. Projected peak gusts of 55-65 mph for most lower elevations, up to 75 mph for wind prone areas and 100-120 mph for Sierra ridges. The first round of precip with this storm moves into the Sierra crest Thursday morning, then increases in intensity as it moves south during the day. Snow levels Thursday around 9000 feet for Mono County. The front will initially not bring that much cooling, with snow levels only edging down to near 8000 feet for Mono by early Friday morning. One final low pressure then moves south across northern CA with rain and snow for most areas Friday-Friday night. The main cold air behind the front will arrive Friday night with more substantial cooling and snow levels dropping to ~ 7000’. Elevations above 8,000 feet could see 1-3’ of additional snowfall by Friday evening.

Two-Day Mountain Weather ForecastProduced in partnership with the Reno NWS

For 8,000 ft. to 10,000 ft.

Tuesday

Tuesday Night

Wednesday

Weather:

Cloudy. Snow and rain.

Cloudy. Rain and snow.

Mostly cloudy. Chance of rain and snow.

Temperatures:

28 to 38 deg. F.

25 to 35 deg. F.

31 to 41 deg. F.

Wind direction:

Southwest

Southwest

Southwest

Wind speed:

30 to 50 mph. Gusts up to 120 mph decreasing to 105 mph in the afternoon.

30 to 40 mph decreasing to 20 to 30 mph after midnight. Gusts up to 90 mph.

15 to 25 mph. Gusts up to 55 mph increasing to 65 mph in the afternoon.

Expected snowfall:

8 to 16 in.

2 to 8 in.

1 to 4 INCHES in.

Over 10,000 ft.

Tuesday

Tuesday Night

Wednesday

Weather:

Cloudy. Snow.

Cloudy. Snow.

Mostly cloudy. Chance of snow.

Temperatures:

.24 to 31 deg. F.

22 to 28 deg. F.

24 to 32. deg. F.

Wind direction:

Southwest

West becoming southwest

Southwest

Wind speed:

60 to 80 mph. Gusts up to 165 mph decreasing to 145 mph in the afternoon.

50 to 70 mph with gusts to 130 mph, decreasing to 40 to 55 mph with gusts to 105 mph after midnight.

30 to 50 mph. Gusts up to 90 mph increasing to 100 mph in the afternoon.

Expected snowfall:

12 to 20 in.

6 to 10 in.

2 to 4 in.

Disclaimer

This Snowpack Summary is designed to generally describe avalanche conditions where local variations always occur. This product only applies to backcountry areas located outside established ski area boundaries. The information in this Snowpack Summary is provided by the Eastern Sierra Avalanche Center, who is solely responsible for its content.

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